Myron's Mopeds

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Break a ring, how not to: Two stroke pistons have little pins (stumps, bumps) in the piston ring grooves that prevent the rings from rotating. Knowing where each ring locating pin is, and aligning the ring gap over the pin during cylinder installation, is the key to not breaking the ring. There’s more about that in the Service Department.

Measuring wear: A new piston ring is a the best tool to measure how worn a cylinder is. Conversely, a new cylinder is the best tool to measure how worn a ring is. To measure the wear of a ring, you need a perfectly round cylinder of a precisely known bore. A worn cylinder that has not been honed will still have it’s original new bore at the very top, above where the rings rub at. A new ring in a new cylinder should have a very small end gap of .004 to .008″. A worn but still OK cylinder with a new ring will have a gap of .0010 to .018″. A worn cylinder and ring both will have an end gap of over .020″(.5mm). A badly worn cylinder and/or ring will have a huge gap, over .030″(.75mm). A feeler gauge is the only way to measure ring gap.

Getting the right rings: If your model is not in the applications, then you must measure the bore and the ring thickness. A calipers is the best tool to measure the bore (ring diameter) and ring thickness. Sometimes your rings might not be original, for many reasons. The piston might be aftermarket with thinner rings. The cylinder might be bored out to an oversize bore. Certain moped year models came with more than one piston size or ring style. For most mopeds, measuring is the only way to be sure of the ring size.  

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It’s important that the piston fits the cylinder properly. An old messed up piston that fits correctly is better than a new piston that fits too loose or too tight. Too loose will be hard to start and weak. Too tight will seize up when hot. Often a cylinder needs to be bored out slightly to fit an oversize piston. Some aluminum cylinders have plated walls that cannot be bored out. The piston diameter, measured at the skirt, is a hair smaller than the cylinder bore. The following piston/cylinder clearance limits are from the Puch factory service manual:

Plated aluminum 7-fin cylinder:  Min .0008“(.020mm)   Max .0016“(.040mm) 

Hi-tor 8-fin, Mag 9-fin cylinders:  Min .001″  (.029mm)    Max .002“(.055mm)

As the engine gets hot, the piston to cylinder clearance approaches zero.

Besides D, H, Ø, S, other things matter in piston compatibility. The ring pins must locate the ring gaps so they never pass over a port. The exhaust-side skirt must be long enough to cover the exhaust port at tdc. The intake-side skirt, on a piston port two stroke, controls the intake timing. Shorter skirt opens sooner and closes later. A longer intake skirt is one way some mopeds are restricted in power. Simply grind the excessively long piston skirt back to 30mph specification, but no farther, to de-restrict the piston. Puch, Tomos, Garelli are the main kinds that use longer pistons to lower the speed, for 25 and 20mph states or jurisdictions. 

This list contains all pistons made by original equipment manufacturers, although most of them are no longer available. The not-available ones are listed for their information, such as speed versions or piston size choices. If you happen to have an unknown piston with or without a part number, you can identify what it is from this list by measuring it and looking for matching measurements. From all these specifications, perhaps a compatible piston can be found or adapted.